The present invention relates in general to a power supply having two DC outputs, and more specifically to a power supply employing a transformer with two secondary windings, one secondary winding being tightly coupled to the primary winding and the other secondary winding being loosely coupled to the primary winding, one DC output voltage being regulated by pulse-width modulation and the other DC output voltage being regulated by frequency modulation.
In power supplies with multiple output voltages, each output voltage is usually regulated by a separate series-pass regulator. This technique has high losses and also a high parts count making it undesirable.
A DC-DC converter is known wherein two individually controlled outputs are obtained by combining a buck circuit and a buck-boost circuit. A transformer is employed having two secondary windings, each tightly-coupled to the primary winding. The buck circuit is connected to one secondary winding and the buck-boost circuit is connected to the other secondary winding. In this converter, the buck circuit is operated such that its output inductor carries continuous current, while the buck-boost circuit is operated such that its output inductor carries discontinuous current. This allows the output voltage of the buck circuit to be controlled by adjusting the duty ratio of the AC signal supplied to the transformer primary winding. The output voltage of the buck-boost circuit is regulated by adjusting the frequency of the AC signal. However, due to the discontinuous inductor current of the buck-boost circuit, its output current is rather limited. Furthermore, since the buck circuit must operate in a continuous inductor current mode, it has a minimum output current under all conditions. Thus, the output voltages of this converter are controlled only within limited output current ranges.
Accordingly, it is a principle object of the present invention to provide a power supply with two independently controllable output voltages which operates in an efficient manner and with a low parts count.
It is a further object of the invention to provide a dual output DC-DC converter with two independently controllable output voltages wherein the output voltages may be regulated over a wide output current range.